1. Field of the Invention
The invention is for the generation of signals for raster scanned video displays employing digital means.
2. Prior Art
With the reduced cost of large scale integrated circuits it has become possible to provide low-cost microcomputers suitable for home use. One such use which has flourished in recent years is the application of microcomputers in conjunction with video displays for games and graphic displays. Most often an ordinary television receiver is employed as the video display means. The standard, raster scanned, cathode ray tubes employed in these receivers and like displays, present unique problems in interfacing these displays with the digital information provided by the microcomputer.
In presenting color graphics it is, of course, desirable to provide high resolution lines and to avoid "ragged" lines. In a microcomputer controlled display, typically a single frequency reference source is employed to generate the color subcarrier reference signal of 3.579545Mhz and the horizontal and vertical synchronization signals. If the frequency of the horizontal synchronization signals is to remain close to its normal frequency (i.e. 15,750hz) the horizontal synchronization means must operate at an odd-submultiple of the color subcarrier frequency. When this occurs there is a phase reversal or phase shift of the color subcarrier reference signal when compared to color control signal between each of the lines of the display. This results in ragged vertical lines unless the color signals are changed for each line. One prior art solution to this problem has been to operate the horizontal synchronization counter at an even submultiple of the color subcarrier frequency (i.e. 15,980hz). This deviation from the standard horizontal synchronization frequency typically requires manual adjustment of the receiver and for some receivers horizontal synchronization may be more difficult to maintain.
As will be described with the invented microcomputer, the horizontal counter operates close to its standard frequency (15,734hz). Through use of a timing compensation means, counting in the horizontal synchronization counter is delayed to compensate for the fact that the counter operates at an odd-submultiple frequency of a color reference signal. In this manner, phase reversal of the color reference signal is eliminated and sharp graphic displays are provided without complex programming.
In many prior art microcomputer controlled displays, color information is stored as four digital bits which are used to designate green, red, blue, and high/low intensity. The color generation means generally includes a signal generator for generating the pure color signals (CW). These pure color signals are then gated and mixed in accordance with the binary state of the four bits to provide a color signal compatible with standard television receivers. Generation of the video color signal in this manner is complex and requires a substantial amount of circuitry.
The invented microprocessor includes a recirculating shift register which circulates four bits of information. In this manner video color signals are generated directly from digital information without the cumbersome generation techniques employed in the prior art.